Samoan Passage Experiment

The Samoan Passage Experiment took place during Sept 92 - Feb 94 in the South Pacific Ocean. The following links take you to pages that describe the experiment and the data it produced and allow you to download the current meter records.

A schematic of the mooring array is shown below as a vertical section running from west (on the left) to east (on the right). To reach a particular current meter, click on its representation in the figure. This will take you to a page of metadata relevant to that instrument, plus descriptive data plots from it.

Client-side image map here

Move up one level.

Description of the Experiment

The Samoan Passage experiment was designed to determine the northward transport of the abysssal water through the Samoan Passage (10S, 170W). This topographic constriction forms the major connection for deep interbasin flow between hemispheres in the Pacific. Here we present current meter data from the six subsurface moorings deployed in the Passage during September 1992 and recovered in February 1994.

The six moorings were placed along a transect of the Passage (see map). A total of 27 Aanderaa RCM8 current meters were employed, each measuring horizontal current and temperature, with the upper two meters on each mooring also measuring pressure. All instruments were recovered. There were some failures:

The quality of the remaining records was excellent.

Samoa objectives contributed to the WOCE international plan; the current meter array has been identified by WOCE as component PCM-11.

The experiment's PIs were R.D.Pillsbury of Oregon State University and Daniel L. Rudnick of University of Washington.


Funds for the Samoan Passage current meter work were provided via Grant OCE-9496015 from the National Science Foundation Physical Oceanography Program, which is gratefully acknowledged. We appreciate the assistance and cooperation given by the masters and crews of the R/V Knorr and R/V Thomas Thompson, and by our seagoing colleagues and companions on the deployment and recovery cruises.


Johnson, G. C., D. L. Rudnick and B. A. Taft (1994): Bottom water variability in the Samoa Passage. J. Mar. Res., 98, 6883-6893.

Roemmich, D., S. Hautala and D. Rudnick (1996): Northward abyssal transport through the Samoan Passage and adjacent regions. J. Geophys. Res., 101, 14039-14055.

Rudnick, D. L. (1997): Direct velocity measurements in the Samoan Passage. J. Geophys. Res., 102, 3293-3302.

Return to the top of this page.

RCM Sampling and Processing Details

All moorings were instrumented with Aanderaa RCM-8 current meters to record speed, direction, and temperature, with some meters equipped to record pressure. (All of the instruments employed RCM-5 savonius rotors and vanes, however.)

The current meter recording interval was set to 60 minutes. During the 60-minute interval the number of rotor revolutions and compass directions were sampled 50 times. Each such sample represents a current vector, its magnitude given by the rotor revolutions and its direction by the compass reading. In the RCM-8 on-board circuitry resolves the vector into two components, eastward and northward. Successive components are added and immedidately stored. When the selected recording interval has elapsed, the resulting average vector magnitude and average angle are calculated and recorded as speed and direction.

Temperature and pressure are instantaneous measurements at the end of the sampling interval.

The current meter stores analogs in the range [0, 1023], rather than the actual speeds, temperatures, etc. In the case of the Samao data the analogs were converted to usable form (speeds in cm/sec, temperatures in degrees, etc.) by means of calibration equations supplied by the OSU Buoy Group's calibration facility. Speeds were calculated by means of a nonlinear equation obtained with the Group's tow tank. The threshold of the Savonius rotor is 1.86 cm/sec. Speeds below the threshold (i.e., analog values of 0) were set to half the threshold - 0.93 cm/sec.

Each current record is archived in three forms: the raw data, exactly as recorded by the instrument, processed data in which the analog values have been cleaned up and converted to metric units, and llp filtered data. The llp (low low passed) file is made from the processed file; it has a time increment of 6 hours and is produced with a 60+1+60 point Cosine-Lanczos filter with half-amplitude at 40 hours and half-power at 46.6 hours. This filter removes the diurnal tides and all higher frequencies.

Occasional problems appear in the data as isolated spikes, gaps, or short runs of unexplainably erratic values. Problem areas only a few data cycles in length are corrected by linear interpolation. Those longer than a few hours but shorter than a week are bridged by predictive interpolation. The latter technique employs Anderson's (1974) algorithm for a predictive filter that utilizes the data on both sides of the gap (Smylie et al, 1973; Ulrych et al, 1973). All corrections to the data are noted elsewhere on this disk.

Predictive interpolation references

Anderson, N. 1974. On the calculation of filter coefficients for maximum entropy spectral analysis. Geophysics 39,69-72.

Smylie, D.E., G.K.C. Clarke and T.J. Ulrych. 1973. Analysis of irregularities in the earth's rotation. Methods in Computational Phys. 13, 391-430.

Ulrych, T.J., D.E. Smylie, O.G. Jensen and G.K.C. Clarke. 1973. Predictive filtering and smoothing of short records by using maximum entropy. J. Geophys. Res. 78, 4959-4964.

Return to the top of this page.

Time series plots, by mooring

To see all of the time series recorded at a particular mooring, click on the name of the mooring below. The moorings lie in a line running from NW to SE; Mooring 1 is at the western end of the line and Mooring 6 is at the eastern end.

Return to the top of this page.